This invention relates in general to systems for particle detection on surfaces and, in particular, to a particle detection system employing a subsystem for collecting light from surfaces within predetermined apertures or collection angles.
In the process of manufacturing devices such as semiconductor wafers, flat panel displays, photomasks, ceramic substrates and other devices, it is important to detect contaminant particles on the surface of an object using the principle of light scattering. In U.S. Pat. No. 4,898,471 assigned to Tencor Instruments, the assignee of the present application, a system for detecting particles and other defects on a patterned semiconductor wafer, photomask or the like is disclosed. A polarizing filter is used in the system to polarize the beam of light in the direction substantially parallel to the surface of the patterned semiconductor wafer to be examined. The beam is enlarged in cross-sectional diameter by a beam expander placed along the path of the beam after the polarizing filter. The beam is then caused to scan by a deflection mirror. The scanning beam is then focused on the patterned wafer at a grazing angle of incidence along an incident direction that is substantially parallel to the patterned streets formed on the wafer. A light collection system for detecting scattered light is positioned in an azimuth direction in a range from about 80.degree.-100.degree. from the incident direction of the scanning beam. The light collection system includes a lens for focusing the scattered light, a polarizing filter oriented in a direction substantially parallel to the surface of the patterned wafer and a photomultiplier tube.
In many semiconductor wafers, such as those formed in manufacturing logic devices, 45.degree. geometries are prevalent, either as interconnects or as repeated feature edges. If the 45.degree. geometries are small in size relative to the wavelength of the incident scanning beam and spot size, the diffraction generated by such geometries can be filtered in the Fourier plane with a programmed blockage. If such geometries are irregularly shaped or are widely spaced apart, it may be difficult to filter the diffraction pattern at the Fourier plane.
Given a known diffraction pattern from the geometries of the pattern on the semiconductor wafer, it may be possible to design a light collection system that would avoid the high intensity pattern scatter in specific directions. However, since the path of the scanning beam covers the entire length or width of the wafer, the scattered light can originate from any point of the long scan line. If the scanning beam has a cross-section that is large relative to the size of the particles being detected, specular reflection and some components of reflection from patterned features would contain many times the light radiated from the particles. Thus the light collection system would have to be designed to avoid the specular reflection and reflection from patterned features originating from any point on the long scan line.
To detect defects on the patterned wafer, one existing technique is to construct templates from the scattered light from individual die of the wafer and comparing the template to the scattered light from other die on the wafer. Both pattern features and contamination have specific and often extremely selective radiation patterns. In order for the comparison to the template to be meaningful, it is very desirable for the light collection system to collect light at a constant collection angle; existing particle detection systems for patterned wafers have used Fourier plane stops to limit the aperture of the system. While this can be achieved for any collection angle, due to the long scan length discussed above, and the need to avoid specular beams from geometries such as the 45.degree. geometries on the wafers, it may be difficult to design a light collection system that is practical. In many instances, the optical system simply becomes too large for users.
From the above, none of the particle detection systems and the light collection subsystems they employ are entirely satisfactory. It is therefore desirable to provide an improved particle detection system and an improved light collection subsystem in which the above-described difficulties are alleviated.